This is the blog of Robert Catterall, an IBM Db2 for z/OS specialist. The opinions expressed herein are the author's, and should not be construed as reflecting official positions of the IBM Corporation.

Tuesday, July 29, 2014

Back in the 1990s, almost all of the DB2 for z/OS-accessing transactional applications I encountered were of the CICS variety. Often, a given DB2 subsystem served as the database manager for multiple CICS-based applications. In such cases, isolation of DB2 accounting information (i.e., database activity figures obtained from DB2 accounting trace records) at the application level was not difficult. DB2 accounting trace records contain multiple identifier fields, and several of these are suitable for separating CICS-DB2 data access information along application lines. For example, one could set things up so that CICS-DB2 application A uses DB2 plan X, while application B uses plan Y. Thereafter, have your DB2 monitor generate an accounting long report (depending on the monitor used, this may be called an accounting detail report) with data aggregated (i.e., ordered or grouped -- again, the term will depend on the DB2 monitor used) at the DB2 plan level, and there's your application-specific view of database activity. You could also have application A run in CICS region X, and application B in region Y, and aggregate information in a DB2 monitor-generated accounting report by region ID (the corresponding DB2 accounting identifier might be referred to as connection identifier by your monitor). It's also easy to separate DB2 accounting information by CICS transaction name (your monitor might call this identifier a correlation name) and by DB2 authorization ID (so, CICS transactions associated with application A might use the DB2 authorization ID X, while transactions associated with application B use the DB2 authorization ID Y). These identifiers are generally usable with online DB2 monitor displays of thread activity as well as with batch-generated accounting reports, and they are as useful for CICS-DB2 applications today as they were 20 years ago.

While CICS-based applications are still a major component of the overall DB2 workload at many sites, lots of organizations have seen access to DB2 from applications running in WebSphere Application Server (WAS) increase at a rapid clip (for some companies, the large majority of DB2 for z/OS-accessing transactions are WAS-based). As is true of organizations that use CICS with DB2, organizations that use WAS with DB2 like to get an application-level view of database activity. How that can be accomplished is the subject of this blog entry.

First, let's look at the simplest situation -- one that's quite common: different WAS-based applications connect to a given DB2 for z/OS system using different authorization IDs. A master data management application might use DB2 authorization ID X, while a customer care application uses ID Y. In that case (as previously mentioned for CICS applications), you have your DB2 monitor generate an accounting long report with data ordered by primary authorization ID, and you're set. Easy. [You can further have your DB2 monitor, if you want, include or exclude data for an accounting report by an identifier such as authorization ID. Additionally, as pointed out already, authorization ID and other DB2 accounting identifiers are generally usable for differentiating data in online monitor displays as well as in monitor-generated reports.]

Sometimes, authorization ID isn't a granular-enough identifier for application-level isolation of DB2 accounting information. That is true when several WAS-based applications connect to a DB2 for z/OS system using the same authorization ID. This situation is not as unusual as you might suppose. More than a few organizations go this route as a means of simplifying security administration. How do these folks get application-specific DB2 accounting information? Let's consider some DB2 accounting data identifier possibilities:

Plan name -- This probably won't do it for you. If you're using the type 4 JDBC driver for your application (the one that is used for programs that access DB2 data via the DB2 for z/OS distributed data facility, aka DDF), all applications will be associated with the same DB2 plan: DISTSERV. If you use the type 2 JDBC driver (an option when WAS is running in the same z/OS LPAR as the target DB2 system), it is possible to provide a differentiating plan name for an application, but in my experience people tend to go with the default plan name of ?RRSAF for all type 2 JDBC-driver using applications. [Note that an application running in WAS for z/OS and accessing a DB2 subsystem on the same z/OS LPAR can use either the type 2 or type 4 JDBC driver.]

Requesting location -- A possibility, yes, but not if your organization runs -- as plenty do -- multiple applications in one instance of WAS. Multi-application WAS instances are particularly common in a z/OS environment, because a) z/OS LPARs often have a very large amount of processing capacity, and b) the sophisticated workload management capabilities of z/OS facilitate the hosting of multiple applications in one LPAR.

Main DB2 package -- Probably not granular enough. WAS-based applications typically issue SQL statements in the form of JDBC calls, and when that's the case the main DB2 package for all applications will be one associated with the JDBC driver.

Transaction name -- More than likely, too granular.

End user ID -- Also too granular, and perhaps not a differentiator if the same end user utilizes several applications.

At this point you might be thinking, "So, what's left?" I'll tell you what identifier fits the bill for numerous organizations that use WAS together with DB2 for z/OS: workstation name. Truth be told, this did not initially occur to me when I pondered the database activity differentiation question in the context of WAS-based applications that use the same DB2 authorization ID. I got hung up on the term "workstation," and thought of that as being an identifier that would be tied somehow to an actual physical device. Silly me. As succinctly and plainly explained by a WAS-guru colleague of mine, "it's just a string" -- a label. And, it's a string that can easily be set for a WAS-based application, through several means:

Via the WAS administration console GUI (in which case it would be an extended property of an application's data source).

Via the IBM Data Server Driver for JDBC (the driver provides a JAR file that contains the DB2Connection class, and that class supports the Java API setDB2ClientWorkstation).

Via application code, for JDBC 4.0 and above (you would use the Java API setClientInfo).

[Note that with regard to the second and third options in the list above, option three (the setClientInfo Java API) is recommended over option two (the Data Server Driver method), because setDB2ClientWorkstation was deprecated with JDBC 4.0.]

Once the workstation name has been set for your WAS-based applications, you can direct your DB2 monitor to generate accounting reports with data ordered by workstation name, and voila -- there's your application-specific view of database activity (and workstation name should also show up in your DB2 monitor's online displays of thread activity).

Want more information on this topic? You can find plenty -- with examples -- in an IBM redbook titled, DB2 for z/OS and WebSphere Integration for Enterprise Java Applications (downloadable at http://www.redbooks.ibm.com/abstracts/sg248074.html?Open). In particular, check out sections 5.5 and 8.2 of this document.

And one more thing: while I've described workstation name as a means of separating DB2 accounting information along the lines of WAS-based applications, you should keep in mind that identifiers provided by Java client information APIs can also be very useful for workload classification in a z/OS WLM policy.

Some DB2 for z/OS people who are relatively new to the client-server application scene may be a little uneasy about such applications, thinking that they can't monitor and control them as they could the DB2 transactional applications of old. In fact, the monitoring and controlling facilities you want are there. Use them, and rest a little easier.

Wednesday, July 16, 2014

A couple of weeks ago, I got a question from a mainframe DB2 DBA about the impact of DB2 buffer pool enlargement on application and system performance. This individual had requested an increase in the size of a buffer pool on his system, and the system administration team had agreed to implement the change; however, one of the system administrators told the DBA that while the larger buffer pool would improve elapsed times for application processes accessing objects assigned to the pool, no CPU efficiency gains should be expected.

I am regularly surprised at the persistence of this notion that bigger DB2 for z/OS buffer pools do not drive CPU savings. Let me see if I can set the record straight in clear terms: YES, THEY DO. I'll explain herein why this is so, and I'll provide a means whereby you can measure the CPU -- yes, CPU -- impact of a DB2 buffer pool size increase.

I don't think that anyone would dispute that a larger buffer pool will decrease I/O activity (especially read I/O activity) for objects (table spaces and/or indexes) assigned to the pool. The disagreement is over the impact of I/O activity on the CPU consumption of DB2-accessing applications, and on the CPU consumption of DB2 itself. What I've found is that some people believe that a System z server's I/O assist processors handle ALL of the processing associated with I/O operations (this view seems to be more widely held by people who have been working with mainframes for a long time, perhaps because I/O assist processors were a more talked-about feature of the platform back in the day). This is not true. I/O assist processors offload from general-purpose engines a substantial portion -- but not all -- of the work involved in reading and writing data from and to disk. I/O assist processors are great, and they are one reason that System z has long excelled as a platform for I/O-intensive applications, but general-purpose engines (and zIIP engines, for that matter) still have to shoulder some of the read/write load.

Thus it is that a reduction in I/O activity will reduce CPU consumption on a mainframe system. If you enlarge a DB2 buffer pool (to reduce disk reads and writes) AND you change that buffer pool to be page-fixed in real storage (via -ALTER BUFFERPOOL bpname PGFIX(YES)), you'll get even more in the way of CPU savings, because one of the things that a general-purpose engine typically has to do in support of a DB2 I/O operation is fix in memory the page holding the DB2 buffer in question (the one into which data will be read into or written from) until the I/O action is complete, after which the page is released (i.e., made pageable again). This is done so that the buffer won't be paged out to auxiliary storage in the middle of the I/O operation. When a pool's buffers are fixed in memory from the get-go (true when PGFIX(YES) is in effect), the page-fix/page-release actions formerly needed for I/Os are not required, and CPU consumption is reduced accordingly. In a DB2 10 or 11 for z/OS system, you can get even more CPU efficiency benefits from page-fixed buffer pools, because in those environments DB2 will request that a page-fixed buffer pool be backed by 1 MB page frames, versus 4 KB page frames (the LFAREA parameter of the IEASYSxx member of PARMLIB specifies the amount of a z/OS LPAR's memory that is to be managed in 1 MB frames). The 1 MB page frames save CPU by improving the efficiency of virtual storage to real storage address translation.

OK, on now to measuring the effect of a buffer pool change (such as enlarging a pool, or page-fixing the buffers in a pool) on application and DB2 CPU efficiency. For the application-level CPU effect, use Accounting Long Reports that can be generated by your DB2 for z/OS monitor (depending on the monitor that you use, these might be called Accounting Detail Reports). Input to these reports is the data contained in records generated when DB2 accounting trace classes 1, 2, and 3 are active (these records are typically written to SMF). With those trace classes active (and BEFORE you've implemented the buffer pool change), do the following:

Generate an Accounting Long Report for a particular day of the week (e.g., Tuesday) and a particular time period. That time period could capture a "peak" of system activity (e.g., 9-11 AM in the morning), or it might be an entire 24 hours -- go with the FROM and TO times that are of interest to you. You can have the DB2 monitor aggregate information in the report in a variety of ways (using an ORDER or GROUP specification -- or something similar, depending on the monitor that you use -- in the report control statement in the SYSIN part of the JCL for the report-generating job). Use the aggregation level (or levels -- you could choose to generate several reports) of interest to you. Want to see the CPU impact on the overall application workload for the DB2 system? Have the data aggregated at the DB2 subsystem level. Want to see the impact for different subcomponents of the workload (e.g., CICS-DB2 work, DRDA work, call attach facility batch work, etc.)? Have the data aggregated by connection type. Note that, by default, a DB2 monitor will typically aggregate accounting information by primary DB2 authorization ID within DB2 plan name -- that is an aggregation that I usually find to be not very useful.

Implement the buffer pool change.

Generate an "after" Accounting Long Report, for the same day of the week (e.g., Tuesday) and the same time period (e.g., 9-11 AM) as for the "before" report. Use the same aggregation specification as before (e.g., at the DB2 subsystem level). Looking at the "before" and "after" reports, find the average in-DB2 CPU time (also known as the average class 2 CPU time), which is the average CPU time for SQL statement execution. Note that this time will be in two fields: general-purpose engine time, and "specialty engine" CPU time (this is typically zIIP engine time). Do NOT overlook the specialty engine time -- for some workloads, particularly the DRDA workload that comes through the DB2 DDF address space, specialty engine CPU time can be greater than general-purpose CPU time. See how these CPU times (general-purpose and specialty engine) have changed, and there's your effect at the application level (the "average" is per DB2 accounting trace record -- one of these is usually generated per online transaction, and per batch job). If you requested that the monitor aggregate data at (for example) the connection type level, you will have in the accounting report a sub-report for each connection type (one for the CICS connection type, one for DRDA, one for call attach, etc.), and there will be an average in-DB2 CPU time (again, both a general-purpose engine and a specialty engine time) in each of these sub-reports.

The procedure for measuring the impact of a buffer pool change on DB2's CPU consumption (i.e., on the CPU time charged to DB2 tasks versus tasks associated with DB2-accessing application programs) is similar to what I described above:

BEFORE making the buffer pool change, use your DB2 monitor to generate a Statistics Long Report for the subsystem (your monitor might refer to this as a Statistics Detail Report). Input to this report is the data in records generated by the "standard" DB2 statistics trace classes (1, 3, 4, 5, and 6). Use the same day of the week and same time period as for the aforementioned Accounting Long Reports.

AFTER making the buffer pool change, generate another Statistics Long Report, for the same day of the week and the same time period as before. In the "before" and "after" reports, find the section of the report in which the CPU times for the DB2 address spaces are provided. Look at the CPU times for the DB2 database services address space (the one most affected by I/O activity -- it handles prefetch reads and database writes), and there's your DB2 CPU impact. I say "look at the CPU times" because you should see both a total CPU time for the address space and a field with a name like "preemptable IIP SRB time." The latter is zIIP engine time, and it is NOT included in the former (reported "total" CPU time is general-purpose engine time).

To summarize this blog entry's message: buffer pool size increases should deliver CPU savings on your system, at both the application level and the DB2 subsystem level, by reducing I/O activity. Those CPU savings can be boosted further by page-fixing pools (usually done most effectively for your higher-activity pools), and page-fixed pools save additional CPU when they are backed by 1 MB page frames (automatic in DB2 10 and DB2 11 environments, when LFAREA in IEASYSxx sets aside some of the LPAR's memory resource to be managed in 1 MB frames). When you've made a buffer pool change that should provide enhanced CPU efficiency for your DB2 applications and subsystem, by all means measure that impact. Your best measurement tool for that purpose is your DB2 monitor, and the Accounting and Statistics Long Reports that it can generate.